remove not used rest_client

1. "json的内容可以通过设置Content-Type为application/json达成，没必要单独增加一个json字段。" remove not used code
2.  在alpaca撤单调试过程中，发生返回值200，但body.text是json的情况，致rest_client解析json报错，加了try/catch做处理

vnpy/api/rest/rest_client.py

@@ -49,10 +49,6 @@ class Request(object):
 
         self.response = None
         self.status = RequestStatus.ready
-        self.json=None
-
-    def set_json(self, json_str: dict):
-        self.json=json_str
 
     def __str__(self):
         if self.response is None:
@@ -65,7 +61,6 @@ class Request(object):
             "headers: {}\n"
             "params: {}\n"
             "data: {}\n"
-            "json: {}\n"
             "response:"
             "{}\n".format(
                 self.method,
@@ -75,7 +70,6 @@ class Request(object):
                 self.headers,
                 self.params,
                 self.data,
-                self.json,
                 "" if self.response is None else self.response.text,
             )
         )
@@ -151,7 +145,6 @@ class RestClient(object):
         on_failed: Callable = None,
         on_error: Callable = None,
         extra: Any = None,
-        json_str: dict = None,
     ):
         """
         Add a new request.
@@ -177,8 +170,6 @@ class RestClient(object):
             on_error,
             extra,
         )
-        if json_str is not None:
-            request.set_json(json_str)
         self._queue.put(request)
         return request
 
@@ -262,7 +253,6 @@ class RestClient(object):
                 headers=request.headers,
                 params=request.params,
                 data=request.data,
-                json=request.json,
                 proxies=self.proxies,
             )
             request.response = response